Recent studies have documented an intricate, patchy mosaic of tactile receptive fields (fractured somatotopy) in the granule and Purkinje cells layers of the mammalian cerebellar cortex. Yet, little is known of the anatomical substrates for cerebellar somatotopy. A research program is proposed that will employ modern anatomical and physiological tools to elucidate the projection patterns of cerebellar afferent inputs in order to begin to fill this void. The experiments will focus on physiologically characterized inputs from the trigeminal brainstem complex, in particular those associated with the mystacial vibrissae (whiskers). Five hypotheses will be tested in normal adult rats: 1: Mossy fibers originating in the trigeminal brainstem complex have predictable projections in multiple regions of cerebellar cortex. Anterograde and retrograde bulk-labeling experiments will determine the projection patterns of mossy fibers arising from the three trigeminal brainstem subnuclei known to have significant cerebellar projections. 2: Functionally distinct mossy fiber projections are spatially segregated in the granule cell layer. Anterograde labeling, monoclonal antibody- revealed reference landmarks, and computer-assisted reconstruction methods will permit quantitative study of the somatotopic organization of mossy fibers originating in specified components of the whisker map in trigeminal subnucleus interpolaris. The homeomorphic relationship between the whiskers and the fiber/cell aggregates in interpolaris will be exploited to compare the projections of groups of cells subserving different whishers. 3: The projections and morphologies of single mossy fibers can be predicted based upon their receptive field location and size, and these projections are somatotopic. This study offers a third level of inquiry into mossy fiber somatotopy and structure-function relationships. Intra- axonal recording and labeling methods will be applied to single mossy fibers of trigeminal brainstem origin. The spatial proximity of the trigeminal brainstem complex to the cerebellum and the use of a novel long- range intracellular marker will also permit analysis of relationships between the termination patterns of individual mossy fibers and the morphology of their cells of origin in the medulla and pons. 4: The termination patterns of functionally identified mossy fibers conform to the fractured somatotopy of their target. Receptive field micromapping in the granule cell layer will be followed by intra-axonal recording and staining of two mossy fibers with receptive fields that correspond to marked cerebellar patches. These studies will again focus on the trigeminal projection to crus I-II and uvular cortex where the whiskers are heavily represented. 5: Connections between granule and Purkinje cells are more frequent within, as opposed to between, functional columns. An anatomical substrate for the vertical congruence of fractured somatotopy between the Purkinje and underlying granule cell layers will be pursued. Physiologically characterized granule cell axons will be stained in vitro and in vivo to determine whether there are more contacts onto Purinje cells via the ascending and within-path segments of granule cell axons than via their extra-patch parallel fiber segments, as suggested by Llinas (1982).